Reports at the time said that among those in Iranian custody were Abu Ghaith, al-Qaeda leader Osama Bin Laden's son Saad Bin Laden and also his security chief Saif al-Adel, sometimes regarded as al-Qaeda's current number three leader.

US officials have long alleged that some of al-Qaeda's more senior members have sought sanctuary in Iran, and have repeatedly accused Tehran of harbouring them.

Tehran has strenuously denied such allegations, although it admits that some may have slipped through its borders.

In February, Iran said it had rounded up more than 500 al-Qaeda members and deported them to their home countries.

Wanted suspect

BBC correspondent Frank Gardner says it is not clear when Abu Ghaith was captured, but he says he has certainly been a key figure in al-Qaeda's leadership.

He has appeared on several video and audio tapes, claiming responsibility for al-Qaeda attacks, and the tapes have a large audience in the Gulf, our correspondent adds.

One of America's most wanted al-Qaeda suspects, Abu Ghaith is a former religious studies teacher, believed to be in his mid-30s, who left Kuwait before the 11 September attacks.

Abu Ghaith reportedly became famous in Kuwait during the Iraqi occupation of the country in 1990, when he gave strong sermons daring the government to attack Baghdad.

He was allegedly banned from preaching in Kuwait after the country's liberation, because his sermons had turned against the government, the constitution and other Arab states.

His citizenship was stripped by Kuwaiti authorities citing "national interest" after an appearance on Qatar-based al-Jazeera television in which he vowed retaliation for US air strikes against Afghanistan.